Low-frequency electrical behavior of dilute conducting fiber composites

Several dilute discontinuous conducting fiber composites are fabricated, and the observed static conductivities and low-frequency permitivities compared with predictions of transport models. Structural features of the materials are examined, and the important parameters incorporated into the predictions. Novel fabrication techniques are employed to produce systems of highly aligned and well-dispersed fibers.;The electrical properties of composites of uniformly sized graphite and nickel-coated graphite fibers in epoxy and polyester resins are determined. Fiber concentration and length are precisely controlled; several aspect ratios are investigated. The distribution of orientations of the included phase is determined with a digitizing image analyzer and compared to a more accurate manual recording of fiber positions. The use of image analysis for structural characterization of transparent matrix composites is evaluated, and some limitations identified.;The conductor/insulator transition is examined; the effects of composite and fiber sizes, concentration, orientation, and processing method are detailed. The apparent conductivity of a composite with a size on the order of several fiber lengths is shown to be an extensive property, dependent upon the specimen size. The dielectric breakdown of insulating gaps in composites is simulated experimentally with a high-field study using polymer films. The complex permittivities of matrix polymers and composites are characterized from 1 to 1000 kHz at temperatures ranging from 80 to 440 K. The composite dielectric constant is shown to be invariant with temperature and frequency when reduced by the matrix resin property.;Dilute approximations, bounding methods and percolation theory are invoked to predict the electrical transport properties of the materials. Composite permittivity is determined to be well-represented by a simple mean-field approximation for composites with a low degree of fiber aggregation. It is shown that extant transport coefficient models for heterogeneous systems fail to predict the conditions for the insulator/conductor transition; an excluded volume method is proposed to determine the fiber concentration at the conductivity transition.